Conductive powder and making process

ABSTRACT

A conductive powder having an organic silicon polymer layer on the surface of each particle and a metal layer enclosing the silicon polymer layer possesses a stronger bond between the particle base and the metal even at elevated temperature and exhibits a high and stable conductivity and heat resistance.

This invention relates to a conductive powder having high conductivitystability even at elevated temperature and a process for preparing thesame.

BACKGROUND OF THE INVENTION

When rubber and plastic articles are used in an electronic applicationwhere conductivity is often required, it is a common practice to mixconductive powder in the rubber and plastic. Metal powders such ascopper powder, nickel powder and silver powder are often used as theconductive powder although they add to the weight of electronicmaterials because of their high specific gravity in the range of 8 to11. This is undesirable because the electronic materials are desired tobe lightweight. Carbon powders such as graphite and carbon black arealso used as the conductive powder although they fail to provideelectronic materials with a resistivity of 10⁻³ Ω-cm order because oftheir own high resistivity.

Of the conductive powders, metallized powder particles having highconductivity which are prepared by coating insulating powder particlesor low conductivity powder particles with a metal have advantagesincluding an increased freedom of choice for the powder material servingas a nucleus. They find a wide range of application as a conductivefiller or a component added to conductive adhesive and anisotropicconductive film. A number of processes have been studied for theproduction of such metallized powder particles, with the process ofusing an electroless plating solution containing a metal salt and areducing agent being industrially used in practice. For example, bycovering surfaces of glass beads with a continuous silver coat, PottersCo. developed a conductive powder exhibiting the electric properties ofsilver despite a low cost and a low specific gravity. As is well knownin the art, this conductive powder was marked in 1978 by ToshibaBarotini K.K. and used in a variety of applications.

However, silver has the problem that it is oxidized or sulfided toincrease its resistivity during long-term storage in a hot humidatmosphere. There is a desire to have a conductive powder havingelectric properties unsusceptible to oxidation or sulfiding like gold.With respect to the nucleus particles, glass beads are inadequate inelectronic materials requiring a high degree of reliability because theglass beads contain a considerable amount of ionic metals such as Na, K,Ca, Mg and Fe in addition to SiO₂. The metallized glass beads haveanother drawback associated with working. When metallized glass beadsare mixed with rubber or plastics, separation can often occur at theinterface between silver and glass beads. This necessitates the use ofsolvents having fire hazard and the use of rubber rolls rather thanmetal rolls.

The separation at the interface between metal and powder particles isstill the outstanding problem in the manufacture of metallized powderparticles. Since the powder particles and the metal have differentinterfacial properties, the metal will separate from the powderparticles due to changes with time or environmental changes (especiallytemperature changes), resulting in a reduced conductivity.

In order to prevent powder particle-metal separation and produce powderparticles having a metal coating closely adhered thereto, the followingprocesses have already been employed. For example, (1) powder particlesare etched to introduce irregularities in their surface to increase thesurface area for improving the metal adhesion. (2) Powder particles aretreated with a silane coupling agent such as a monomeric silane,typically γ-aminopropyltriethoxysilane for improving the metal adhesion.(3) Powder particles are treated with an organic resin such as an epoxyresin for improving the metal adhesion. See JP-A 59-182961.

However, in method (1), the degradation of powder particles by etchingis a problem. In method (2), powder particles will agglomerate owing toalkoxysilyl groups. In either case, no satisfactory metallized powder isobtained. In method (3), the organic resin with which powder particlesare treated can be decomposed or degraded at elevated temperatures sothat the conductive powder deteriorates its conductive properties. Forexample, where the rubber or plastic article which is required to impartconductivity is a highly heat resistant silicon polymer, that is,silicone rubber, this article cannot be used in the temperature range of150 to 250° C., under which service the silicone rubber itself fullywithstands, because the conductive properties are degraded.

SUMMARY OF THE INVENTION

An object of the invention is to provide a conductive powder havingimproved heat resistance, conductivity and conductivity stability and aprocess for preparing the same.

It has been found that by treating powder particles of silica or thelike with an organic silicon polymer, especially an organic siliconpolymer with reducing effect, typically having Si—Si bonds or Si—H bondsin a molecule, to cover the particle surface with a coat of the siliconpolymer, treating the powder particles covered with the silicon polymercoat with a solution of a metal salt in the presence or absence of asurfactant to deposit metal colloid on the surface of the siliconpolymer coat, and treating the powder particles with an electrolessplating solution to deposit a coat therefrom, there are obtainedmetallized powder particles of the structure that particle surfaces arecovered with the silicon polymer coat, on which the metal is carried andthe metal coat is formed. The process is inexpensive and simple. Theadhesion of the metal coat is very strong. The metallized powderparticles can find use as fillers and anti-fungus agents havingconductivity and catalysis.

The metal plating layer can be formed as a multilayer metal layerincluding a first metal layer typically of nickel and a second metallayer typically of gold overlying the first metal layer. Moreimprovements are then made in heat resistance, conductivity andconductivity stability.

In general, organic silicon polymers are very interesting because oftheir heat resistance, flexibility, and thin film forming capability aswell as the metallic nature and electron delocalization of silicon ascompared with carbon. In particular, silicon polymers having Si—Si bondsor Si—H bonds, especially polysilanes or polysiloxanes having hydrogenatoms directly attached to silicon atoms are known as polymers havingreducing effect and used in a variety of applications. It is also knownthat both polysilane serving as a precursor to silicon carbide ceramicmaterial and polysiloxane serving as a precursor to silicon oxideceramic material can be heat or otherwise treated into an insulatingmaterial having high heat resistance.

We already found that when a substrate is treated with an organicsilicon polymer having reducing action and dipped in a metalion-containing solution, metal ions are reduced on the substrate surfacewhereby the resulting metal colloid is carried on the substrate. Whileutilizing this colloid formation as a catalyst, electroless plating iseffected. Then, a substrate having a metal coat firmly adhered theretois produced.

Utilizing the above-described nature of the silicon polymer, we havedeveloped a powder exhibiting stable conductive properties even atelevated temperatures and a process for producing the same. Electrolessplating on powder particles, especially substantially ionic metal-freepowder particles, is enabled by previously forming a layer of organicsilicon polymer having reducing effect on the powder particles. By theelectroless plating, powder particles are coated with a first metallayer and then with a second metal layer. Especially when an oxidationresistant metal is used as the second metal layer, little or no loss ofconductivity occurs even at elevated temperatures. Final heat treatmentresults in metallized powder particles featuring a strong bond betweenthe particle base and the metal layer. The resulting conductive powdercan be compounded in heat resistant rubber such as silicone rubberwithout losses of its properties, so that the resulting compound isuseful as a raw material for manufacturing reliable connectors andgaskets.

We have further found that by treating particles of silica or the likewith an organic silicon polymer having reducing effect to form a siliconpolymer layer on the particle surface, treating the particles with asolution containing a salt of a metal having a standardoxidation-reduction potential of at least 0.54 volt, thereby depositingcolloid metal on the silicon polymer layer, effecting electroless nickelplating and then gold plating on the particles, there is obtained aconductive powder in which an organic silicon polymer layer, anickel-phosphorus alloy layer, and a gold layer are formed successivelyon the particle surface. When the electroless nickel plating involveselectroless nickel plating in a first electroless nickel platingsolution having a first phosphorus reducing agent concentration, andelectroless nickel plating in a second electroless nickel platingsolution having a second phosphorus reducing agent concentrationdifferent from the first concentration, the resulting nickel-phosphorusalloy layer has a phosphorus content which differs between inner andouter surface regions and especially which is lower in the outer surfaceregion than in the inner surface region. The metallized powder has afirmly bonded gold layer and high conductivity, and is heat resistantenough to prevent the plating layer from separating even in heattreatment above 200° C. The conductive powder can be compounded insilicone rubber etc., from which reliable, highly conductive rubberparts can be manufactured.

More specifically, we have found that in the manufacture of conductivepowder, a nickel-phosphorus alloy layer having a high phosphorus contentis more adhesive to powder particles of silica or the like whereas anickel-phosphorus alloy layer having a low phosphorus content is moreamenable to displacement plating with gold. By effecting electrolessnickel plating initially at a low pH and in the presence of an excessiveamount of reducing agent relative to a nickel salt concentration, thecontent of phosphorus in nickel can be increased to enhance the adhesionto silica. Thereafter, an aqueous solution containing a nickel salt, acomplexing agent and a pH adjuster is replenished to the platingsolution to raise the pH thereof whereby plating is effected in thepresence of a short amount of reducing agent relative to the nickel saltconcentration. The reduced phosphorus content in nickel facilitates goldplating. There is produced a conductive powder of the four layerstructure consisting of particle base-silicon polymer-nickel/phosphorusalloy-gold and featuring tight adhesion therebetween.

According to a first aspect of the invention, there is provided aconductive powder in which an organic silicon polymer layer and a metallayer are successively formed on surfaces of particles. The metal layerpreferably includes a first metal layer and a second metal layer.

According to a second aspect of the invention, there is provided aconductive powder in which a partially or entirely ceramic layer oforganic silicon polymer and a metal layer are successively formed onsurfaces of particles. The metal layer preferably includes a first metallayer and a second metal layer.

According to a third aspect of the invention, there is provided aprocess for preparing a conductive powder, comprising the steps oftreating particles each having a surface with an organic silicon polymerhaving reducing effect to form a silicon polymer layer on the particlesurface; treating the particles with a salt of a metal having a standardoxidation-reduction potential of at least 0.54 volt, thereby depositinga colloid of the metal on the organic silicon polymer layer; andthereafter, treating the particles with an electroless plating solution,thereby depositing a metal layer on the outermost surface of theparticles.

According to a fourth aspect of the invention, there is provided aprocess for preparing a conductive powder, comprising the steps of:

(1) treating particles each having a surface with an organic siliconpolymer having reducing effect to form an organic silicon polymer layeron the particle surface,

(2) treating the particles with a solution containing a salt of a metalhaving a standard oxidation-reduction potential of at least 0.54 volt,thereby depositing a colloid of the metal on the organic silicon polymerlayer,

(3) effecting electroless plating on the particles with the metalcolloid serving as a catalyst, to deposit a first metal layer on theouter surface of the organic silicon polymer layer, and

(4) effecting plating on the particles to form a second metal layer onthe first metal layer.

The process may further include the step of heat treating the particlescovered with the metal layer at a temperature of at least 150° C. forconverting at least a part of the organic silicon polymer into aceramic.

In one preferred embodiment, the first metal layer is selected fromamong nickel, copper, silver, cobalt, tungsten, iron and zinc, and thesecond metal layer is selected from among gold, platinum, and palladium.More preferably, the first metal layer is nickel, and the second metallayer is gold. Then the desirable metallized powder has a four layerstructure consisting of particle base-silicon polymer-nickel-gold. Goldis preferable as the second metal layer presenting the outermost surfaceof the conductive powder because gold has the highest conductivity amongnoble metals and does not undergo a rise of resistivity due to oxidationand sulfiding during long-term storage in a hot humid atmosphere. Nickelis preferable as the first metal layer because it is characterized by alow cost, corrosion resistance and appropriate hardness and serves as astable underlying layer for bearing the second metal layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an instrument for measuring the conductivity of aconductive powder.

FIG. 2 illustrates a grinder type instrument for an adhesive force.

FIGS. 3(A) to 3(C) are cross-sectional photomicrographs of gold-platedsilica of Example 4.

FIGS. 4(A) to 4(C) are cross-sectional photomicrographs of gold-platedsilica of Example 5.

DETAILED DESCRIPTION OF THE INVENTION

The conductive powder of the invention is based on particles each havinga surface on which an organic silicon polymer layer and a metal layerare successively formed.

The powder particles used herein include those of inorganic and organicmaterials which can accommodate electroless plating. The shape ofparticles may be spherical, rod, needle, hollow or irregular. Allparticulate materials which are handled as particles or powder on outerappearance are included. Such particles may have a size of about 0.001μm to several mm, and desirably a mean particle size of 0.01 μm to10,000 μm.

The inorganic powder particles used herein are typically inorganicfillers which are conventionally used as an extending filler,reinforcing filler or colorant in plastics and rubber. Include are metalpowders, metal or non-metal oxides, metal silicates includingaluminosilicates, metal carbides, metal nitrides, metal acid salts,metal halides, and carbon. Examples include silica, alumina, aluminumsilicate, talc, mica, shirasu balloon, graphite, glass fibers, siliconfibers, carbon fibers, asbestos, potassium titanate whiskers, zincwhite, aluminum nitride, magnesium oxide, boron nitride, nickel powderand aluminum powder. The organic powder particles used herein includeparticles of insulating resins such as phenolic resins, polyesterresins, epoxy resins, polyamide resins, polyimide resins, acrylic esterresins, acrylonitrile resins, urethane resins, polyacetal resins, alkydresins, melamine resins, silicone resins, fluoro-resins, polyethyleneresins, polypropylene resins, polybutene resins, polystyrene resins,polyvinyl chloride resins, poly(diaryl phthalate) resins, polyxyleneresins, polyvinyl alcohol, and polycarbonate, as well as particles oflow conductive resins such as polyaniline resins, polyacetylene resins,polythiophene resins and polypyrrole resins.

For use in electronic materials which require high reliability,inorganic powder particles which do not contain ionic metals and arestable in heat resistance are desirable. Silica is most desirable sinceit is compatible with silicon polymers. Silica is available as particlesof silicon dioxide and is readily obtained by burning chlorosilane orwater glass, spraying and firing an emulsion of hydrolyzed alkoxysilane,oxidizing gasified metallic silicon, or fusing quartz powder. Silica maytake the form of powder, fibers or flakes while its shape is notcritical. However, silica particles of spherical shape providing aminimum specific surface area among different shapes with an identicalparticle diameter is preferred in order to minimize the amount ofplating metal (e.g., nickel or gold) used and ensure high loading inresins or rubber. Spherical particles which do not have surface-openpores in the interior are especially preferred in order to reduce thespecific surface area. Fused quartz powder is useful in this respect.

Silica preferably has a particle size of 0.01 to 1,000 μm, morepreferably 0.1 to 100 μm, and further preferably 1 to 50 μm. A particlesize of less than 0.01 μm provides an increased specific surface area,which may increase the amount of plating metal used, leading to aneconomic disadvantage. Particles with a size in excess of 1,000 μm maybe difficult to mix with resins or rubber.

According to the invention, the powder particles are covered on theirsurface with an organic silicon polymer (or siliceous high molecularweight compound) in the first step. The silicon polymers used herein arepreferably those having reducing effect, including polysilanes,polycarbosilanes, polysiloxanes, and polysilazanes having Si—Si bondsand/or Si—H bonds, especially polysilanes, as well as polysiloxaneshaving hydrogen atoms directly attached to silicon atoms.

One preferred silicon polymer having Si—Si bonds in the molecule is apolysilane, which is preferably represented by the following generalformula (1):

(R¹ _(m)R² _(n)X_(p)Si)_(q)  (1)

wherein R¹ and R² each are hydrogen or a substituted or unsubstitutedmonovalent hydrocarbon group; X is a group as defined for R¹, alkoxygroup, halogen atom, oxygen atom or nitrogen atom; m, n and p arenumbers satisfying 0.1≦m≦2, 0.1≦n≦1, 0≦p≦0.5, and 1≦m+n+p≦2.5, and q isan integer of 4≦q≦100,000.

In formula (1), R¹ and R² are independently hydrogen or substituted orunsubstituted monovalent hydrocarbon groups. R¹ and R² may be the sameor different. The monovalent hydrocarbon groups are aliphatic, alicyclicor aromatic ones. Preferred aliphatic or alicyclic monovalenthydrocarbon groups are those of 1 to 12 carbon atoms, especially 1 to 6carbon atoms, for example, alkyl groups such as methyl, ethyl, propyl,butyl, pentyl and hexyl and cycloalkyl groups such as cyclopentyl andcyclohexyl. Preferred aromatic monovalent hydrocarbon groups are thoseof 6 to 14 carbon atoms, especially 6 to 10 carbon atoms, for example,phenyl, tolyl, xylyl, naphthyl and benzyl. Substituted monovalenthydrocarbon groups are those of the foregoing unsubstituted monovalenthydrocarbon groups in which some or all of the hydrogen atoms arereplaced by halogen atoms, alkoxy groups, amino groups, and aminoalkylgroups, for example, monofluoromethyl, trifluoromethyl, andm-dimethylaminophenyl.

X is a group as defined for R¹, alkoxy group, halogen atom, oxygen atomor nitrogen atom. Exemplary alkoxy groups are those of 1 to 4 carbonatoms such as methoxy, ethoxy, and isopropoxy. Exemplary halogen atomsare fluorine, chlorine and bromine atoms. Among others, X is preferablymethoxy or ethoxy.

Letters m, n and p are numbers satisfying 0.1≦m≦2, preferably 0.5≦m≦1;0≦n≦1, preferably 0.5≦n≦1; 0≦p≦0.5, preferably 0≦p≦0.2; and 1≦m+n+p≦2.5,preferably 1.5≦m+n+p≦2. Letter q is an integer of 4≦q ≦100,000,preferably 10≦q≦10,000.

The preferred silicon polymer having hydrogen atoms directly attached tosilicon atoms (Si—H groups) is a polysiloxane having Si—H groups on sidechains and Si—O—Si bonds on the backbone. Such polysiloxane isrepresented by the following general formula (2):

(R³ _(a)R⁴ _(b)H_(c)SiO_(d))_(e)  (2)

wherein R³ and R⁴ each are hydrogen, a substituted or unsubstitutedmonovalent hydrocarbon group, alkoxy group or halogen atom; a, b, c andd are numbers satisfying 0.1≦a≦2, 0≦b≦1, 0.01≦c≦1, 0.5≦d≦1.95, and2≦a+b+c+d≦3.5, and e is an integer of 2≦e≦100,000.

In formula (2), R³ and R⁴ are independently hydrogen, substituted orunsubstituted monovalent hydrocarbon groups, alkoxy groups or halogenatoms. R³ and R⁴ may be the same or different. The monovalenthydrocarbon groups are aliphatic, alicyclic or aromatic ones. Preferredaliphatic or alicyclic monovalent hydrocarbon groups are those of 1 to12 carbon atoms, especially 1 to 6 carbon atoms, for example, alkylgroups such as methyl, ethyl, propyl, butyl, pentyl and hexyl andcycloalkyl groups such as cyclopentyl and cyclohexyl. Preferred aromaticmonovalent hydrocarbon groups are those of 6 to 14 carbon atoms,especially 6 to 10 carbon atoms, for example, phenyl, tolyl, xylyl,naphthyl and benzyl. Substituted aliphatic, alicyclic or aromaticmonovalent hydrocarbon groups are those of the foregoing unsubstitutedmonovalent hydrocarbon groups in which some or all of the hydrogen atomsare replaced by halogen atoms, alkoxy groups, amino groups, andaminoalkyl groups, for example, monofluoromethyl, trifluoromethyl, andm-dimethylaminophenyl. Exemplary alkoxy groups are those of 1 to 4carbon atoms such as methoxy, ethoxy, and isopropoxy. Exemplary halogenatoms are fluorine, chlorine and bromine atoms. Among others, methoxy orethoxy is preferred.

Letters a, b, c and d are numbers satisfying 0.1≦a≦2, preferably0.5≦a≦1; 0≦b≦1, preferably 0.5≦b≦1; 0.01≦c≦1, preferably 0.1≦c≦1;0.5≦d≦1.95, preferably 1≦d≦1.5; and 2≦a+b+c+d≦3.5, preferably2≦a+b+c+d≦3.2. Letter e is an integer of 2≦e≦100,000, preferably10≦e≦10,000.

In the first step according to the invention, the above-described powderparticles are treated with an organic silicon polymer to form an organicsilicon polymer layer on the particle surface. To this end, the siliconpolymer is dissolved in an organic solvent. The particles are pouredinto the solution or vice verse. After mixing, the organic solvent isremoved whereby a silicon polymer layer is formed on the particlesurface.

The organic solvents in which the silicon polymer is dissolved includearomatic hydrocarbon solvents such as benzene, toluene and xylene,aliphatic hydrocarbon solvents such as hexane, octane and cyclohexane,ether solvents such as tetrahydrofuran and dibutyl ether, esters such asethyl acetate, aprotic polar solvents such as dimethylformamide,dimethyl sulfoxide and hexamethyl phosphoric triamide, nitromethane andacetonitrile.

The solution preferably has a concentration of 0.01 to 50% by weight,more preferably 0.01 to 30% by weight, most preferably 0.1 to 10% byweight of the silicon polymer. With a too low concentration, theresulting silicon polymer layer would become too thin and difficult toform evenly on the particle surface. A large amount of the solvent usedmay invite a cost increase. With a too high concentration, the resultingsilicon polymer layer would become thick so that the coated particlesmight tend to agglomerate.

As the preferred treating technique, a slurry of the solution having thesilicon polymer dissolved therein and the particles are brought intodispersing contact by rotating an agitation blade in a container.Thereafter, the solvent is removed by filtration or distilling off atelevated temperature or in vacuum. A spraying technique of dispersingthe slurry in a gas stream for achieving instantaneous drying is alsopreferable. Drying is also achieved by agitating the slurry at atemperature above the boiling point of the solvent and a suitablepressure, which technique is effective for preventing agglomeration. Inthe above treating step, the organic solvent is distilled off at anelevated temperature or under vacuum. The most effective drying is byagitating the slurry at a temperature above the boiling point of thesolvent, for example, at a temperature of about 40 to 200° C. under avacuum of 1 to 100 mmHg.

At the end of treatment, the treated particles are held for a while in adry atmosphere or at a temperature of 40 to 200° C. under vacuum. Duringthe period, the solvent is effectively distilled off and the treatedparticles are dried. In this way, silicon polymer-coated particles areobtained.

The silicon polymer layer generally has a thickness of 0.001 to 1.0 μm,desirably 0.01 to 0.1 μm. With a thickness of less than 0.001 μm, it maydifficult to form a silicon polymer layer uniformly on the particlesurface, leaving some surface areas uncovered. A thickness of more than1.0 μm, that is, a thick silicon polymer layer may cause the coatedparticles to agglomerate and increase the amount of silicon polymerused, which is economically disadvantageous.

In the second step according to the invention, the siliconpolymer-treated powder particles resulting from the first step aretreated with a metal salt. This is done by bringing the surface of thesilicon polymer-treated powder particles in contact with a solution ofthe metal salt. In this treatment, owing to the reducing action of thesilicon polymer, metal colloid deposits on the surface of the siliconpolymer coat.

It is known in connection with electroless plating that a conductivepowder is obtained by surface treating particles with a solutioncontaining noble metal ions such as palladium to form a noble metallayer on the particle surface, and contacting the treated particles witha plating solution containing metal ions, whereby the metal deposits incoating form on the surface of the noble metal layer from the platingsolution, thereby forming a uniform, firmly adhering metal coating onthe particle surface. The noble metal layer has a catalytic function ofpromoting the deposition of metal ions on the particle surface from theplating solution in the subsequent step.

In order that noble metal ions be firmly captured on the particlesurface and play the role of catalytic nuclei in the subsequent step, itis a common practice to surface treat the particle surface with asurface treating agent capable of capturing noble metal ions (see JP-A59-182961.)

However, simply capturing noble metal ions is insufficient. Since noblemetal ions are dissolvable in water, the noble metal can be dissolvedout as ions into the plating solution in the subsequent step whereby itsfunction as catalytic nuclei is aggravated. It is thus proposed in JP-A1-242782 to add a reducing agent to the plating solution used in thesubsequent step whereby the noble metal ion layer which has simplydeposited on the particle surface is converted into a colloid which canfunction as uniform complete catalytic nuclei.

In contrast, the present invention utilizes the reducing action of thesilicon polymer covering the particle surface so that a metal colloiddeposits from the metal ion to form a robust metal layer which functionsas catalytic nuclei.

The metal salt from which the metal colloid is formed is a salt of ametal having a standard oxidation-reduction potential of at least 0.54volt. Specifically salts of gold having a standard oxidation-reductionpotential of 1.50 volts, palladium having a standard oxidation-reductionpotential of 0.99 volt, and silver having a standard oxidation-reductionpotential of 0.80 volt are preferably used. Salts of metals having astandard oxidation-reduction potential of lower than 0.54 volt, forexample, copper having a standard oxidation-reduction potential of 0.34volt and nickel having a standard oxidation-reduction potential of 0.25volt are difficult to be reduced by the silicon polymer.

The gold salts are those of Au⁺ or Au³⁺, for example, NaAuCl₄,NaAu(CN)₂, and NaAu(CN)₄. The palladium salts are those of Pd²⁺ and aregenerally represented by Pd—-Z₂ wherein Z is a halogen such as Cl, Br orI, acetate, trifluoroacetate, acetylacetonate, carbonate, perchlorate,nitrate, sulfate, or oxide. Examples are PdCl₂, PdBr₂, PdI₂,Pd(OCOCH₃)₂, Pd(OCOCF₃)₂, PdSO₄, Pd(NO₃)₂, and PdO. The silver salts arethose which generate Ag⁺ when dissolved in a solvent and are generallyrepresented by Ag—Z wherein Z is perchlorate, borate, phosphate orsulfonate. Examples are AgBF₄, AgClO₄, AgPF₆, AgBPh₄, Ag(CF₃SO₃), andAgNO₃.

As the contact technique of treating the powder particles with asolution of the metal salt, it is preferred to prepare a solution of themetal salt in a solvent which does not dissolve the silicon polymer, butcan dissolve or disperse the metal salt, and admit the siliconpolymer-coated powder particles into the solution whereby the particlesare contacted with the metal salt. With this treatment, the metal saltis adsorbed and simultaneously reduced on the surface of the siliconpolymer layer covering powder particles, forming metal-coated powderparticles.

Examples of the solvent which does not dissolve the silicon polymer, butcan dissolve or disperse the metal salt include water, ketones such asacetone and methyl ethyl ketone, alcohols such as methanol and ethanol,and aprotic polar solvents such as dimethylformamide, dimethyl sulfoxideand hexamethyl phosphoric triamide, with water being especiallypreferred.

The concentration of the metal salt varies with the solvent in which thesalt is dissolved and preferably ranges from 0.01% by weight to the saltsaturated solution. A concentration of less than 0.01% may provideinsufficient catalysis for plating. Beyond the saturated solution, thesolid salt will undesirably precipitate out. When the solvent is water,the concentration of the metal salt is preferably 0.01 to 20%, and morepreferably 0.1 to 5% by weight.

Typically, the silicon polymer-treated powder particles are immersed ina solution of the metal salt at room temperature to 70° C. for about 0.1to 120 minutes, more preferably about 1 to 15 minutes. Then there isobtained a powder in which metal colloid has deposited on the surface ofthe silicon polymer layer.

According to the invention, the treatment with the metal salt solutionis effected either in the presence or absence of a surfactant, desirablyin the presence of a surfactant. More particularly, as a result of thesilicon polymer treatment, the particles are hydrophobic. The siliconpolymer-coated particles have less affinity to the metal salt solutionand are not readily dispersed therein, indicating that the reaction offorming metal colloid is inefficient. It is thus recommended to add asurfactant to improve the dispersion, ensuring that the siliconpolymer-coated particles are briefly dispersed in the metal saltsolution.

The surfactant used herein may be any of anionic surfactants, cationicsurfactants, ampholytic surfactants, and nonionic surfactants.

The anionic surfactants include sulfonic salts, sulfuric ester salts,carboxylic salts, and phosphoric ester salts. The cationic surfactantsinclude ammonium salts, alkylamine salts and pyridinium salts. Theampholytic surfactants include betain, aminocarboxylic acid, and amineoxide surfactants. The nonionic surfactants include ether, ester andsilicone surfactants.

Illustratively, the anionic surfactants include alkylbenzenesulfonicacid salts, sulfosuccinic acid esters, polyoxyethylene alkyl sulfatesalts, alkyl phosphate esters, and long chain fatty acid soaps. Thecationic surfactants include alkyl chloride trimethylammonium salts,dialkyl chloride dimethylammonium salts, and alkyl chloride pyridiniumsalts. The ampholytic surfactants include betain sulfonic salts andbetain aminocarboxylic amine salts. The nonionic surfactants includepolyoxyethylene alkyl ethers, polyoxyethylene fatty acid esters andpolyoxyalkylene-modified polysiloxanes. A commercially available aqueoussolution of such surfactant, for example, Mama Lemon® (Lion Corporation)is also useful.

The amount of the surfactant added is preferably such that the siliconpolymer-coated particles may be uniformly dispersed in the metal saltsolution containing a surfactant or a surfactant solution. Desirably0.0001 to 10 parts, more desirably 0.001 to 1 part, and especially 0.01to 0.5 part by weight of the surfactant is used per 100 parts by weightof the metal salt solution. Less amounts of the surfactant are lesseffective whereas excessive amounts of the surfactant may adverselyaffect the covering power of subsequent plating and cause discolorationof the plated metal.

As the treating technique involving the use of a surfactant, it isrecommended that the silicon polymer-treated powder particles are firstcontacted with the surfactant alone or the surfactant diluted withwater, dispersed by agitation, and then contacted with the metal saltsolution. With this order, the reducing action of the silicon polymerpromotes the reaction of forming the metal on the silicon polymer layersurface.

Where the surfactant is omitted, the silicon polymer-treated powderparticles are contacted with the solvent and dispersed therein bythorough agitation.

After the above treatment, the treated particles are treated with afresh solvent of the same type, but free of the metal salt for removingthe unnecessary metal salt which has not been carried on the particles.Finally the unnecessary solvent is dried off from the particles,yielding a metal-coated powder. Drying is preferably effected at 0 to150° C. under atmospheric pressure or vacuum.

According to the invention, the next step is to effect electrolessplating on the metal-coated powder. By effecting electroless platingwhile the metal colloid resulting from the second step serves as acatalyst, powder particles covered completely with a variety of metalscan be produced. The metal layer may be a single layer or plural layers,and preferably consists of a first metal layer (or underlying metallayer) and a second metal layer overlying the first metal layer.

The electroless plating solution used to form the first metal layer maybe selected from a variety of well-known solutions. The preferred metalsto be added to the plating solution are metal materials containing, forexample, nickel, copper, silver, cobalt, tungsten, iron and zinc. Thesemetals may be used alone or as alloys thereof, such as Ni—Co, Ni—W,Ni—Fe, Co—W, Co—Fe, Ni—Cu, and Ni—P. When it is desired to form an alloycoating, a plurality of metal salts may be added. Nickel is especiallypreferred.

In addition to the metal salt(s), the electroless plating solutiongenerally contains a reducing agent such as sodium hypophosphite,hydrazine dimethylamine boron, or sodium boron hydride and a complexingagent such as sodium acetate, phenylenediamine or sodium potassiumtartrate. Electroless plating solutions containing copper, nickel,silver and gold are commercially available at a reasonable cost.

The electroless nickel plating solution is described in detail.Generally, the electroless nickel plating solution contains awater-soluble nickel salt, complexing agent, pH adjuster, and reducingagent.

The nickel salt used herein may be selected from well-known salts suchas nickel sulfate, nickel chloride and nickel acetate. The nickel saltconcentration may be 0.5 to 0.01 mol/l, and preferably 0.2 to 0.05 mol/lof the entire plating bath. A too high nickel salt concentration has thedrawbacks that a slight change of pH or a slight change of complexingagent concentration may cause formation of hydroxide to reduce the lifeof the bath and that replenishment tends to invite a local variation ofthe nickel salt concentration which in turn, causes formation of spots.A too low nickel salt concentration may require a greater amount ofreplenishment solution so that the volume of the bath largely changesduring plating, which is impractical.

The complexing agent may be selected from well-known complexing agentsused in electroless nickel plating solutions of this type. Included areammonium salts of inorganic acids such as ammonium chloride, phosphoricsalts, carboxylic acids and water-soluble salts thereof such as sodiumacetate, hydroxycarboxylic acids and water-soluble salts thereof such asammonium citrate and sodium tartrate, and amines having amino andcarboxyl groups and water-soluble salts thereof such as glycine andEDTA. They may be used alone or in admixture of two or more. Thecomplexing agent is not limited to these examples. Of these complexingagents, the hydroxycarboxylic acid salts such as ammonium citrate andsodium tartrate, carboxylic acid salts such as sodium acetate, andamines having amino and carboxyl groups such as glycine are preferablebecause they do not allow nickel hydroxide to form even when the pH ofthe plating bath changes and because they do not form with nickel acomplex ion which is stable enough to prevent reductive deposition ofnickel. The concentration of the complexing agent is closely correlatedto the concentrations of the nickel salt and pH adjuster and usually inthe range of 1.5 to 0.03 mol/l and preferably 0.2 to 0.15 mol/l of theentire plating bath. A larger amount of the complexing agent which isexcessive relative to the nickel salt is wasteful. A smaller amount ofthe complexing agent may invite instability to a pH change and be lesseffective for restraining formation of nickel hydroxide.

As the pH adjuster, any of well-known, inexpensive, readily availableagents may be used. Ammonium hydroxide (aqueous ammonia) and alkalihydroxides such as sodium hydroxide are preferably used because they donot largely alter the reduction of nickel complex ion by the phosphorusreducing agent relative to a pH change. The concentration of the pHadjuster may be determined in accordance with a change of pH of theplating bath during the process, treating time, and replenishmentamount.

The pH of the electroless nickel plating solution is selected dependingon the type of the reducing agent and complexing agent. In the case ofhypophosphorous acid or salt as the reducing agent, the solution isgenerally set at pH 3 to 10. Below pH 3, little plating reaction occurs.Beyond pH 10 may render the nickel complex unstable, and allow forprecipitation of nickel hydroxide and degradation of the platingsolution. Since the pH lowers with the progress of electroless nickelplating, the pH adjuster is preferably replenished so as to keep aconstant pH. It is also recommended to adjust the bath such that thebath may be at pH 3 to 4, especially pH 4 to 5 at the start of platingand at pH 6 to 10, especially pH 6 to 7 at the end of plating.

The reducing agent used herein is preferably a phosphorus reducing agentwhich is selected from hypophosphorous acid and alkali metal andammonium salts thereof, typically sodium hypophosphite. A boron reducingagent such as dimethylamine boron is also preferably used. Theconcentration is desirably 0.1 to 5 mol, more desirably 0.5 to 3 mol ofthe reducing agent per mol of the nickel salt and 2.5 to 0.001 mol/l,more desirably 1.0 to 0.1 mol/l of the entire plating bath.

According to the invention, treatment with the electroless platingsolution can be effected in the presence of a surfactant if necessary.In this embodiment, the metal-coated powder particles are pretreatedwith the surfactant prior to pouring into the electroless platingsolution, or the surfactant is poured into the electroless platingsolution prior to plating treatment. This avoids the undesirablephenomenon that bubbles of hydrogen gas evolving during electrolessplating reaction prevent the effective progress of electroless platingand exacerbate the covering power of the metal to the particle surface.Then the powder particles coated with the silicon polymer can beuniformly surface coated with the metal film.

The surfactant used in electroless plating may be the same as ordifferent from the surfactant used in the previous metal salt treatment(treatment with the metal salt solution). It is preferable to use adifferent surfactant. For example, better results are expected when ananionic surfactant such as sodium alkylbenzenesulfonate is used forimproving the contact with the metal salt solution and a nonionicsurfactant such as polyoxyethylene fatty acid ester orpolyoxyalkylene-modified polysiloxane is added prior to the electrolessplating. More illustratively, those surfactants which act to reducesurface tension, but do not help foaming are desirable. Nonionicsurfactants such as Surfynol 104, 420 and 504 (Nisshin Chemical IndustryK.K.) are preferably used. A lowering of plating efficiency by foamingcan be avoided by adding an anti-foaming surfactant having ananti-foaming effect and reducing surface tension, for example, apolyether-modified silicone surfactant commercially available as KS-538from Shin-Etsu Chemical Industry Co., Ltd.

The amount of the surfactant added is similarly selected such that themetal-coated powder particles may be uniformly dispersed in theelectroless plating solution and the sticking of bubbles of hydrogen gasevolving with the progress of electroless plating to the powderparticles to restrain the electroless plating may be avoided. Desirably,the surfactant is used in an amount of 0.0001 to 10 parts, moredesirably 0.001 to 1 part, most desirably 0.01 to 0.5 part by weight per100 parts by weight of the electroless plating solution. A less amountof the surfactant may be less effective whereas an excessive amount mayadversely affect the covering power of plating metal and causediscoloration of the plated metal.

The plating temperature may be in the range of 15 to 100° C., desirablyin the range of 40 to 95° C. which allows metal ions to diffuse at ahigher rate in the bath, improves the covering power of plating metal,and reduces the loss of bath components and the loss of solvent byvolatilization, and more desirably in the range of 65 to 85° C. Attemperatures below 40° C., plating reaction may proceed very slowly,which is impractical. At temperatures above 95° C., bath control maybecome difficult because of vigorous evaporation of the solvent which iswater, and plating reaction may proceed too fast, causing abnormaldeposition and bath decomposition.

As the plating technique, the powder particles resulting from the secondstep or the water dispersion of the powder particles may be added to theelectroless plating solution, or inversely, the electroless platingsolution may be added to the water-dispersed powder particles. The keyis to ensure that metal depositing reaction proceeds substantiallysimultaneously on powder particles. When the particles or waterdispersion thereof are added to the electroless plating solution, it isrecommended that the solution is kept at a so low temperature that noplating reaction occurs, the entire particles are dispersed in thesolution in a non-agglomerated state, and plating is thereaftercommenced as by heating. Also, when the electroless plating solution isadded the powder particles, it is recommended that the particles arepreviously dispersed in water or the like in a non-agglomerated state,and the plating solution is brought into close contact with theparticles as by agitation whereby plating takes place. In the case, theelectroless plating solution may be a solution obtained by premixing aplating metal salt solution, reducing agent solution, pH adjustersolution and complexing agent solution. Alternatively, the respectivecomponents are separately added to form the electroless plating solutionin situ. Besides the procedure, all the powder particles are dispersedin a solution containing a reducing agent, pH adjuster and complexingagent, and only the metal salt solution carried by a gas stream is addedto the dispersion whereupon electroless plating takes place.

For producing nickel-coated powder particles without agglomeration, itis very effective to disperse the powder particles in a solutioncontaining a reducing agent, pH adjuster and complexing agent, keep thisdispersion at an appropriate temperature for nickel plating, carry anaqueous nickel salt solution on a gas stream, and add the gas-bornenickel salt solution to the dispersion of powder particles. With thehelp of the carrier gas, the aqueous nickel salt solution is quickly anduniformly dispersed in the aqueous solution containing the reducingagent, pH adjuster and complexing agent whereby the powder particles aresurface plated with nickel. Although the introduction of gas ofteninvites a loss of plating efficiency due to foaming, this inconveniencecan be avoided by adding an anti-foaming surfactant. Useful to this endis a surfactant having an anti-foaming effect and reducing surfacetension, for example, a polyether-modified silicone surfactantcommercially available as KS-538 from Shin-Etsu Chemical Co., Ltd.

In the case of electroless nickel plating, the oxygen concentration ofthe plating solution affects the deposition of nickel. The deposition ofnickel is restrained by the presence of more dissolved oxygen becausecolloidal palladium serving as plating catalyst nuclei can be oxidizedinto palladium cations and dissolved out in the solution, and the oncedeposited nickel surface can be oxidized. Inversely, in the presence ofless dissolved oxygen, the plating solution loses stability and nickelcan deposit on areas other than the powder particles, resulting information of fine nickel powder masses and bulbous deposits. It is thusdesirable to maintain the dissolved oxygen in the plating solution at aconcentration of 1 to 20 ppm. Above 20 ppm, the plating rate may dropand uncovered spots be left. Below 1 ppm, formation of bulbous depositsis sometimes observed.

Therefore, it is preferable to use as the gas a mixture of anoxygen-containing gas such as air and an inert gas such as argon ornitrogen. In the case of plating on powder particles, the start ofplating is often delayed, but there is the risk that once plating isstarted, the reaction runs away. In one effective procedure forpreventing the risk, nitrogen is initially used and after the start ofnickel plating reaction is confirmed, the gas is changed to air.

Where electroless nickel plating is effected using a phosphorus reducingagent, the properties of a nickel film can be controlled by controllingthe plating procedure because phosphorus is incorporated into nickel.For example, the powder particles are added to an aqueous solutioncontaining a nickel salt, reducing agent, pH adjuster and complexingagent and held at a temperature of 25° C. at which substantial nickelplating does not take place, the system is fully dispersed, and thenheated to an optimum temperature of 40 to 95° C. for nickel plating.This procedure enables to effect nickel plating without substantiallychanging the proportion of the reducing agent concentration relative tothe nickel salt concentration in the plating solution and the pH of theplating solution. Then a nickel layer having a phosphorus content whichis substantially equal between inner and outer surface regions isdeposited on the powder particles.

It is also possible to effect nickel plating by dispersing the powderparticles of the second step in an aqueous solution of a phosphorusreducing agent and adding thereto an aqueous solution containing anickel salt, and complexing agent and a pH adjuster solution, andchanging the proportion of the reducing agent concentration relative tothe nickel salt concentration in the plating solution and the pH of theplating solution. Then a nickel layer having a phosphorus content whichis different between inner and outer surface regions is deposited on thepowder particles. In this embodiment, there may be furnished first andsecond plating solutions having different contents of the phosphorusreducing agent, and plating in the first plating solution be followed byplating in the second plating solution. Alternatively, after plating iseffected in the first plating solution, an aqueous solution containingone or more of a nickel salt, complexing agent and pH adjuster, but notthe phosphorus reducing agent is replenished to form the second platingsolution in situ where second plating is effected. The nickel salt,completing agent and pH adjuster used for this replenishment arepreferably the same as used in the first plating solution. Thereplenishment is not limited to one and may be repeated two or threetimes.

In the embodiment wherein electroless nickel plating in a firstelectroless nickel plating solution is followed by electroless nickelplating in a second electroless nickel plating solution having adifferent phosphorus reducing agent concentration from the firstsolution whereby a nickel-phosphorus alloy layer having a phosphoruscontent which is different between inner and outer surface regions isdeposited on the powder particles, it is preferred that the phosphorusreducing agent concentration of the first electroless nickel platingsolution is higher than the phosphorus reducing agent concentration ofthe second electroless nickel plating solution so that the inner surfaceregion of the nickel-phosphorus alloy layer has a high phosphoruscontent and the outer surface region has a low phosphorus content. Moreparticularly, the first plating solution has a phosphorus reducing agentconcentration of 2.5 to 0.05 mol/l, especially 1.5 to 0.1 mol/l of theentire plating solution whereby the inner surface region has aphosphorus content of 5 to 20%, especially 8 to 15%. The second platingsolution has a phosphorus reducing agent concentration of 1.0 to 0mol/l, especially 0.5 to 0 mol/l of the entire plating solution wherebythe outer surface region has a phosphorus content of 0.5 to 8%,especially 1 to 5%. The nickel-phosphorus alloy layer preferably has athickness of 0.01 to 10 μm, especially 0.05 to 2 μm though the thicknessis not critical. Of this thickness, a region extending from the outersurface to at least 0.1 μm, especially to 0.5 μm is preferably thesecond nickel-phosphorus alloy layer.

In the third step, the plating time varies with the desired thickness ofthe plating film and is not particularly limited. Usually the platingtime is 1 minute to 16 hours, preferably 5 to 120 minutes, and morepreferably 10 to 60 minutes.

In this way, the first metal layer is formed on the powder particles.According to the invention, a second metal layer is advantageouslyformed so as to enclose the first metal layer. In this embodiment, thethird step of forming the first metal layer is followed by the fourthstep of forming the second metal layer as quickly as possible becausethe second metal layer should be formed before the first metal layer isoxidized.

The fourth step is to effect further plating on the powder particlesresulting from the third step to form a second metal layer on the firstmetal layer, yielding a conductive powder. The method for forming thesecond metal layer or surface film on the powder particles having thefirst metal layer already formed thereon may be electroless plating,electroplating or displacement plating. In the case of electroplating,it is necessary to use a cathode configured to a special shape so thatthe metal may uniformly deposit on the powder particles and a systemdesigned such that the powder particles may be periodically separatedfrom the cathode. This step may also employ displacement plating thatutilizes substitution reaction between the first and second metal layersat their interface due to a difference in ionization tendencytherebetween.

Of these methods, the electroless plating method is advantageouslyemployed in the fourth step.

The electroless plating solution used to form the second metal layer maybe a solution prepared by the same method as above. The preferred metalsto be added to the plating solution are gold, platinum and palladium.These metals may be used alone or as alloys thereof, such as Au—Pd,Au—Pt, and Pd—Pt. Of these, gold is especially preferred for stabilityand conductivity. Such a gold plating solution is on the market andreadily available. The plating solutions generally include cyanide,chloride and sulfite solutions although the commercially easilyavailable cyanide solutions are preferred mainly for the economicreason. The electroless plating method may be the same as in the thirdstep.

The plating temperature may be in the range of 15 to 100° C., desirablyin the range of 40 to 95° C. which allows metal ions to diffuse at ahigher rate in the bath, improves the covering power of plating metal,and reduces the loss of bath components and the loss of solvent byvolatilization, and more desirably in the range of 65 to 85° C. Attemperatures below 40° C., plating reaction may proceed very slowly,which is impractical. At temperatures above 95° C., bath control maybecome difficult because of vigorous evaporation of the solvent which iswater, and plating reaction may proceed too fast, causing abnormaldeposition and bath decomposition. An appropriate plating time is 1minute to 16 hours, preferably 5 to 120 minutes, and more preferably 10to 60 minutes.

At the end of the fourth step, thorough washing is carried out to removethe unnecessary metal salt, reducing agent, complexing agent, surfactantand other additives.

In each of the second to fourth steps, it sometimes happens that somespots of the surface of each particle are left uncovered due to theagglomeration of particles. The agglomeration means that many powderparticles gather together due to the secondary force. Since individualparticles remain independent and not associated with each other, theyseparate by a slight force. If the silicon polymer-treated powderparticles are in an agglomerated state in the second step, the metalcolloid cannot deposit within agglomerates. If an agglomerated stateexists in the third and fourth steps, the plating metal cannot depositwithin agglomerates. In either case, there result powder particles whichare partially uncovered with metal, failing to develop excellentconductivity.

To prevent such inconvenience, it is desirable in each step that thepowder particles be dispersed in a solution without agglomeration. Thedispersing method may use an agitator having an agitating blade drivenby a motor or a homogenizer having a rotor for effecting agitation withthe aid of sonic energy.

The thus obtained conductive powder consists of metal-coated powderparticles having the four-layer structure of base particle-siliconpolymer-first metal layer-second metal layer.

The first metal layer has a thickness of 0.01 to 10.0 μm, and preferably0.1 to 2.0 μm. A thickness of less than 0.01 μm may fail to completelycover the particle surface or to provide satisfactory hardness andcorrosion resistance. A thickness of more than 10.0 μm may beeconomically disadvantageous because a corresponding larger amount ofthe first metal increases the cost and specific gravity. The secondmetal layer has a thickness of 0.001 to 1.0 μm, and preferably 0.01 to0.1 μm. A thickness of less than 0.001 μm may allow oxidation in a hothumid atmosphere, which results in an increased resistivity and hence,unstable conductivity. A thickness of more than 1.0 μm may beeconomically disadvantageous because a corresponding larger amount ofthe expensive second metal increases the cost and specific gravity.

If desired, the metal-coated powder particles are finally heat treatedat a temperature of at least 150° C. in an atmosphere of an inert gas(e.g., argon, helium or nitrogen) or a reducing gas (e.g., hydrogen,argon-hydrogen or ammonia). The inert or reducing gas heat-treatingconditions include a temperature of 150° C. to 900° C. and a time of 1minute to 24 hours and desirably a temperature of 200° C. to 500° C. anda time of 30 minutes to 4 hours. This heat treatment converts part orall of the silicon polymer between the particle base and the metallayers into a ceramic so that the particles has higher heat resistance,insulation and adhesion. When the atmosphere is a reducing atmospheresuch as hydrogen, the oxide in the metal layers can be decreased and thesilicon polymer be converted into a stable structure whereby the powderparticles possessing a stronger bond between particle base (typicallysilica) and metal and exhibiting higher conductivity are obtained. It isnoted that if polysilane is treated at such a high temperature, Si—Sibonds are severed, allowing various elements to be incorporated thereinfor stabilization. As a result, heat treatment in an oxidizingatmosphere such as air produces silicon oxide ceramics, heat treatmentin a reducing atmosphere such as ammonia gas produces silicon nitrideceramics, and heat treatment in an inert atmosphere such as argon orvacuum produces silicon carbide ceramics.

The conductive powder of the invention finds a wide range of applicationas a conductive filler. For example, the conductive powder is blended inrubber compositions such as silicone rubber compositions or resincompositions to impart high conductivity thereto.

EXAMPLE

Examples of the invention are given below by way of illustration and notby way of limitation.

Synthetic Example 1

Preparation of Polysilane

Phenylhydrogenpolysilane (abbreviated as PPHS) was prepared by thefollowing procedure. In an argon-purged flask, a diethyl ether solutionof methyl lithium was added to a THF solution ofbis(cyclopentadienyl)dichlorozirconium. After 30 minutes of reaction atroom temperature, the solvent was distilled off in vacuum whereby acatalyst was formed within the system. Phenyltrihydrosilane was added tothis in an amount of 10,000 mol per mol of the catalyst. The contentswere heated and agitated for 3 hours at 100 to 150° C. and then for 8hours at 200° C. The product was dissolved in toluene and washed withaqueous hydrochloric acid whereby the catalyst was deactivated andremoved. Magnesium sulfate was added to this toluene solution to removewater. By filtration, PPHS having a weight average molecular weight of1,200 and a glass transition temperature of 65° C. was collected in asubstantially quantitative yield.

EXAMPLE 1

Treatment of Silica with Silicon Polymer (1st Step)

The silica used herein was prepared by classifying spherical silica(Mitsubishi Rayon K.K., mean particle size 10 μm, specific surface area0.4 m²/g), cutting off a fraction of particles having a mean particlesize of less than 1 μm, and collecting silica particles with a meanparticle size of 12 μm and a specific surface area of 0.28 m²/g. This isdesignated SiO-12.

A solution of 0.5 g of PPHS in 65 g of toluene was added to 100 g ofSiO-12, which was agitated for one hour into a slurry. Toluene andsilica were separated by filtration. To remove toluene more completely,the silica was dried in a rotary evaporator at a temperature of 80° C.and a vacuum of 45 mmHg while rotating. The PPHS-treated sphericalsilica was disintegrated by means of a roll mill or jet mill. This isdesignated PPHS-treated SiO.

Preparation of Palladium Colloid-deposited Silica (2nd Step)

The polysilane-treated silica (PPHS-treated SiO) would float on thesurface of water since it was hydrophobicized. 5 g of the treated silicawas admitted into 10 g of a 0.5% aqueous solution of surfactant Surfynol504 (Nisshin Chemical Industry K.K.) whereby the silica was dispersed inwater. This is designated water dispersed PPHS-treated SiO.

For palladium treatment, 7 g of a 1% PdCl₂ aqueous solution (0.07 g ofpalladium chloride, 0.04 g of palladium) was added to 15 g of the waterdispersed PPHS-treated SiO, followed by 30 minutes of agitation. By thistreatment, palladium colloid adhered to the silica surface. Theresulting silica was colored blackish gray.

The silica was separated from the aqueous palladium solution byfiltration, washed with water, and immediately poured into 10 g of anaqueous solution containing a surfactant (0.1% aqueous solution ofanti-foaming agent KS-538 by Shin-Etsu Chemical Co., Ltd.) fordispersing in water. This is designated water-dispersed PPHS-treatedSiO—Pd.

Preparation of Nickel-plated Silica (3rd Step)

A plating solution was prepared by diluting 20 ml of an aqueous solutionof 0.2 mol/l nickel sulfate, 20 ml of an aqueous solution of 0.4 mol/lsodium hypophosphite and 20 ml of an aqueous solution of 0.4 mol/l ofsodium hydroxide, 5 ml of an aqueous solution of 0.05 mol/l sodiumcitrate, and 10 ml of an aqueous solution of 0.1 mol/l sodium acetatewith ion-exchanged water to 100 ml. With stirring, 15 g of thewater-dispersed PPHS-treated SiO—Pd was dispersed in 100 ml of theplating solution at 25° C. to form a plating solution. The solutiontemperature was quickly raised to 70° C. by immersing the container in awater bath at 70° C. After a while, fine bubbles started to evolve andthe solution turned its color from deep green to blackish green.

Silica particles having metallic nickel deposited on the entire surfacewere obtained. The plated silica was separated from the plating solutionby filtration, washed with pure water, and isolated by filtration again.While the silica remained wet, it was poured into 10 g of an aqueoussolution containing a surfactant (0.1% aqueous solution of anti-foamingagent KS-538 by Shin-Etsu Chemical Co., Ltd.) whereby the silica wasdispersed in water. This is designated water-dispersed nickel-coatedsilica.

Preparation of Gold-plated Silica (4th Step)

The gold plating solution used was 120 ml of a gold plating solutionK-24N containing cyanoaurate (Kojundo Chemical Laboratory Co., Ltd.).With vigorous stirring, the entire amount of the water-dispersednickel-coated silica was added to the gold plating solution. Thesolution temperature was raised from room temperature to 85° C., andimmediately thereafter, the silica turned golden, indicating that thenickel on the silica surface was replaced by gold.

The plated silica was subjected to filtration, water washing and drying(at 50° C. for 30 minutes) and thereafter, fired at 250° C. for one hourin a hydrogen-purged electric furnace.

Properties of Conductive Silica Having Silica-siliconPolymer-nickel-gold Structure

By observation under a stereomicroscope, it was found that the silicaparticles were covered with gold over their entire surface. The platedsilica surface was observed under an electron microscope to find that asmooth and uniform metal film was formed. Upon IPC analysis, thegold-plated silica was found to contain 30 wt % of nickel and 6 wt % ofgold.

The electric resistivity of the gold-plated silica was determined byintroducing the gold-plated silica into a cylindrical cell having fourterminals, conducting a current of -−10 mA to 10 mA across the terminalsof 0.2 cm² area at the opposed ends from a current source SMU-257(Keithley Co.), and measuring a voltage drop across the 0.2 cm spacedapart terminals at the center of the cylinder by means of a nanovoltmeter model 2000 (Keithley Co.).

The measuring instrument is shown in FIG. 1. There are illustratedgold-plated silica 1, gold-plated copper members 2, insulators 3 ofsilicone rubber, a current source 4, and a volt meter 5. Provided that ρis a resistivity, S is the area of the electrode, L is the distancebetween electrodes, R is an electric resistance, I is the currentconducted across the sample, and V is the measured voltage, theresistivity represented by ρ=RS/L is determined from the electricresistance R=V/I. If the instrument is designed such that S is X cm² andL is Y cm wherein X=Y. then the resistivity ρ is equal to R Ω-cm.

In this way, the electric resistivity of the gold-plated silica wasdetermined to be 3.8 mΩ-cm.

After a thermal aging test (250° C., 1 hour in air), the gold-platedsilica had an acceptable electric resistivity of 4.7 mΩ-cm.

The gold-plated silica was admitted into a powder peeling tester. Anadhesion test was carried out by operating the tester at 1,100 rpm forone minute. The silica was examined to find no change of its outerappearance and resistivity. Additional adhesion tests were carried outby operating the tester at 1,100 rpm for 5 and 10 minutes. The silicawas examined to find that its outer appearance turned somewhat black andthe resistivity changed to 5.1 mΩ-cm and 8.9 mΩ-cm, respectively. Whenobserved under a microscope, the interface between gold and nickel waspartially peeled so that the black nickel was exposed, but no peel wasfound at the interface between silica and nickel.

The powder peeling tester used herein is a grinder type peeling forcemeasuring instrument as shown in FIG. 2. The instrument includes acontainer 12 which is filled with gold-plated silica 11 and closed witha rubber packing 16. A motor 13 has a rotating shaft 14 having anabrasive-bearing bit 15 attached to the tip thereof. The bit 15 isinserted into the silica 11 and rotated at a predetermined speed. Thehollow cavity of the container 12 which is filled with the gold-platedsilica has a volume of 0.68 cm³. The abrasive-bearing bit 15 has adiameter of 3 mm. The silica fill is 1.0 g.

Comparative Example 1

For comparison purposes, Example 1 was repeated except that thepolysilane PPHS was omitted. Plating did not take place at all. Nometallized silica was obtained.

Comparative Example 2

For comparison purposes, Example 1 was repeated except that thepolysilane PPHS was omitted and instead, treatment was effected with 100g of a 1% ethanol solution of γ-aminopropyltriethoxysilane (KBE-903 byShin-Etsu Chemical Co., Ltd.). Plating partially took place, butentirely metallized silica was not obtained.

Comparative Example 3

For comparison purposes, Example 1 was repeated except that thepolysilane PPHS was omitted and instead, treatment was effected with 100g of a 1% ethanol solution of γ-aminopropyltriethoxysilane (KBE-903 byShin-Etsu Chemical Co., Ltd.), and palladium colloid reduced withstannous chloride as disclosed in JP-A 59-182961 was used instead of thepalladium chloride. Effective plating took place. Metallized silica wasobtained, but some particles agglomerated.

This metallized silica had a low resistivity of 19.5 mΩ-cm. The silicawas admitted into the powder peeling tester, which was operated at 1,100rpm for one minute. The silica was examined to find that its outerappearance turned whiter and the resistivity became worse in excess of 1Ω-cm. A microscopic observation showed that the silica-nickel interfacewas partially peeled to expose the underlying silica.

EXAMPLE 2

In the third step of Example 1, a plating solution was prepared bydiluting 20 ml of an aqueous solution of 0.4 mol/l sodium hypophosphiteand 20 ml of an aqueous solution of 0.4 mol/l of sodium hydroxide, 5 mlof an aqueous solution of 0.05 mol/l sodium citrate, and 10 ml of anaqueous solution of 0.1 mol/l sodium acetate with ion-exchanged water to100 ml. With stirring, 15 g of the water-dispersed PPHS-treated SiO—Pdwas dispersed in 100 ml of the plating solution to form a platingsolution. The solution temperature was quickly raised to 70° C. byimmersing the container in a water bath at 70° C.

To this was added 1 ml of an aqueous solution of 0.2 mol/l nickelsulfate. After a while, fine bubbles started to evolve and the palegreen solution turned clear. Sequentially, 19 ml of an aqueous solutionof 0.2 mol/l nickel sulfate was slowly added dropwise to the platingsolution which was being agitated, whereby plating reaction took place.Otherwise, the procedure was the same as in Example 1. Silica particlescovered with gold over their entire surface were obtained as in Example1.

Identification of Conductive Silica having Silica-siliconPolymer-nickel-gold Structure

The gold-plated silica was fixedly buried in a resin piece, which wassliced by a diamond cutter. The cross section was observed under atransmission electron microscope H9000NAR (Hitachi, Ltd.), finding atwo-layer structure consisting of a silica portion and a multi-phasemetal portion. The constituent elements were analyzed in a depthdirection to find a multi-layer structure consisting of a gold layer, anickel layer, and a silicon layer stacked in the depth direction. Withrespect to the phosphorus content of the nickel layer, a region of thenickel layer disposed adjacent the silica had a high phosphorus content(of about 8 to 12 wt %), and a region of the nickel layer disposedadjacent the gold had a low phosphorus content (of about 1 to 4 wt %),indicating that the nickel layer had a graded composition structure.

The gold-plated silica had a resistivity of 3.1 mΩ-cm. After a thermalaging test (250° C., 1 hour in air), the gold-plated silica had anacceptable electric resistivity of 4.1 mΩ-cm.

The gold-plated silica was admitted into a powder peeling tester, whichwas operated at 1,100 rpm for one minute. The silica was examined tofind no change of its outer appearance and resistivity. Additionaladhesion tests were carried out by operating the tester at 1,100 rpm for5 and 10 minutes. The silica was examined to find that the resistivitychanged to 4.1 mΩ-cm and 5.9 mΩ-cm, respectively, indicating high peelresistance.

EXAMPLE 3

In the third step of Example 1, the water-dispersed PPHS-treated SiO—Pdwas first dispersed in 100 ml of pure water by agitation. The solutiontemperature was quickly raised to 70° C. by immersing the container in awater bath at 70° C. Then 20 ml of an aqueous solution of 0.2 mol/lnickel sulfate, 20 ml of an aqueous solution of 0.4 mol/l sodiumhypophosphite and 20 ml of an aqueous solution of 0.4 mol/l of sodiumhydroxide, 5 ml of an aqueous solution of 0.05 mol/l sodium citrate, and10 ml of an aqueous solution of 0.1 mol/l sodium acetate weresimultaneously added dropwise to the solution whereby plating reactiontook place. Otherwise, the procedure was the same as in Example 1.Silica particles covered with gold over their entire surface wereobtained.

A cross section of the gold-plated silica was observed as in Example 2,finding a multi-layer structure consisting of a gold layer, a nickellayer, and a silicon layer. The constituent elements were analyzed in adepth direction to find that the nickel layer had an approximately equalphosphorus content of about 12 to 19 wt % in a depth direction,indicating that no graded structure was formed.

The gold-plated silica had an initial resistivity of 3.5 mΩ-cm. After athermal aging test (250° C., 1 hour in air), the gold-plated silica hadan acceptable electric resistivity of 4.5 mΩ-cm.

The gold-plated silica was admitted into a powder peeling tester, whichwas operated at 1,100 rpm for one minute. The silica was examined tofind no change of its outer appearance and resistivity. Additionaladhesion tests were carried out by operating the tester at 1,100 rpm for5 and 10 minutes. The silica was examined to find that the resistivitychanged to 7.5 mΩ-cm and 8.9 mΩ-cm, respectively.

Synthetic Example 2

Preparation of Phenylpolysilane

In an argon-purged flask, a diethyl ether solution of methyl lithium wasadded to bis(cyclopentadienyl)dichlorozirconium, thereby preparingbis(cyclopentadienyl)dimethylzirconium serving as a catalyst in thesystem. Phenylsilane was added to the system in an amount of 50 mol permol of the catalyst, which was heated and agitated at 150° C. for 24hours. The catalyst was then removed by adding a molecular sieve andeffecting filtration, and solid PPHS having a weight average molecularweight of 2,600 was collected in a substantially quantitative yield.

EXAMPLE 4

Treatment of Silica with Silicon Polymer

The silica used herein was spherical silica US-10 (Mitsubishi RayonK.K., mean particle size 10 μm, specific surface area 0.4 m²/g). Asolution of 1 g of PPHS in 65 g of toluene was added to 100 g of US-10,which was agitated for one hour into a slurry. Toluene and silica wereseparated by filtration. To remove toluene more completely, the silicawas dried in a rotary evaporator at a temperature of 80° C. and a vacuumof 45 mmHg while rotating. The PPHS-treated spherical silica wasdisintegrated by means of a roll mill or jet mill.

Preparation of Palladium Colloid-deposited Silica

The PPHS-treated silica would float on the surface of water since it washydrophobicized. 100 g of the PPHS-treated spherical silica was admittedinto 50 g of a 0.5% aqueous solution of surfactant Surfynol 504 (NisshinChemical Co., Ltd.) whereupon the silica was dispersed in water byagitation.

For palladium treatment, 70 g of a 1% PdCl₂ aqueous solution (0.7 g ofpalladium chloride, 0.4 g of palladium) was added to 150 g of thesilica-water dispersion, followed by 30 minutes of agitation, filtrationand water washing. By this treatment, palladium colloid adhered to thesilica surface, and so the silica was colored blackish gray. Thepalladium colloid-deposited silica was isolated by filtration, washedwith water, and immediately subjected to plating.

Nickel Plating of Palladium Colloid-deposited Silica

With stirring, 10 g of the palladium colloid-deposited silica wasdispersed along with 0.5 g of anti-foaming agent KS-538 (Shin-EtsuChemical Co., Ltd.) in 500 ml of a plating solution at 50° C. which hadbeen diluted with ion-exchanged water so as to contain 0.3 mol/l ofnickel sulfate, 0.36 mol/l of ammonium citrate and 0.36 mol/l ofammonium hypophosphite. The solution temperature was quickly raised to85° C. by immersing the container in a water bath at 85° C. After awhile, fine bubbles started to evolve and the solution turned its colorfrom deep green to blackish green and begun to lower its pH from 4. Then500 ml of a mixed solution of 0.3 mol/l nickel sulfate and 0.72 mol/lammonium citrate and 20 ml of an aqueous solution of about 8 vol %ammonia were slowly replenished to the plating solution which was beingstirred, so as to adjust the solution at pH 6. This took 60 minutes.

The plated silica was separated from the plating solution by suctionfiltration, washed with pure water, and isolated by suction filtrationagain. While the silica remained wet, it was transferred to thesubsequent step.

Gold Plating of Nickel-plated Silica

The gold plating solution used was 10 g of a gold plating solution K-24Ncontaining cyanoaurate (Kojundo Chemical Laboratory Co,. Ltd.). Thesilica particles having metallic nickel deposited on their entiresurface were dispersed in ion-exchanged water. With vigorous stirring,the gold plating solution was added dropwise to the dispersion. Thesolution temperature was raised from room temperature to 45° C., andimmediately thereafter, the silica turned golden, indicating that thenickel on the silica surface was replaced by gold.

The plated silica particles settled on the bottom and were subjected tofiltration, water washing and drying (at 50° C. for 30 minutes) andthereafter, fired at 250° C. for one hour in a hydrogen-purged electricfurnace. The resulting silica particles were observed under astereomicroscope to find that silica particles were covered with goldover their entire surface. On IPC analysis of this silica, palladium,nickel and gold were detected.

Identification of Conductive Silica having Silica-siliconPolymer-nickel/phosphorus Alloy-gold Structure

The gold-plated silica was fixedly buried in a resin piece by themicrotome technique, which was sliced by a diamond cutter. The crosssection was observed under a transmission electron microscope H9000NAR(Hitachi, Ltd.), finding a two-layer structure consisting of a silicaportion and a multi-phase plated portion. The constituent elements wereanalyzed in a depth direction to find a multi-layer structure consistingof a gold layer, a nickel-phosphorus alloy layer, and a silicon layerstacked in the depth direction. With respect to the phosphorus contentof the nickel-phosphorus alloy layer, a region of the nickel layerdisposed adjacent the silica had a high phosphorus content, and a regionof the nickel layer disposed adjacent the gold had a low phosphoruscontent, indicating that the nickel layer had a graded compositionstructure.

FIGS. 3(A) to 3(C) are cross-sectional photomicrographs on which pointsof elemental analysis are marked. Table 1 shows the contents (wt %) ofSi, P, Ni, and Au at these points of analysis. Table 3 shows thecontents (wt %) of P and Ni at these points of analysis.

Properties of Conductive Silica having Silica-siliconPolymer-nickel/phosphorus Alloy-gold Structure

The electric resistivity of the gold-plated silica was determined byintroducing it into a cylindrical cell having four terminals, conductinga current of 1 mA to 10 mA across the terminals of 0.2 cm² area at theopposed ends from a current source SMU-257 (Keithley Co.), and measuringa voltage drop across the 0.2 cm spaced apart terminals at the center ofthe cylinder by means of a nanovolt meter model 2000 (Keithley Co.). Thegold-plated silica had a resistivity of 2.2 mΩ-cm. The gold-platedsilica was milled in a mortar for one minute and then heat treated at200° C. for 4 hours whereupon it was examined to find no change of itsouter appearance and resistivity.

EXAMPLE 5

Example 4 was repeated except that in the nickel plating step, thepalladium colloid-deposited silica was dispersed in a mixture of anickel plating reducing agent solution and a nickel metal salt solutionwhereupon nickel plating reaction took place. A cross section of thegold-plated silica was similarly observed, finding a multi-layerstructure consisting of a gold layer, a nickel-phosphorus alloy layer,and a silicon layer. The constituent elements were analyzed in a depthdirection to find that the nickel-phosphorus alloy layer had anapproximately equal phosphorus content of about 12 to 16 wt % in a depthdirection, indicating that no graded structure was formed.

FIGS. 4(A) to 4(C) are cross-sectional photomicrographs on which pointsof elemental analysis are marked. Table 2 shows the contents (wt %) ofSi, P, Ni, and Au at these points of analysis. Table 3 shows thecontents (wt %) of P and Ni at these points of analysis.

The gold-plated silica had an initial resistivity of 4.5 mΩ-cm. Thegold-plated silica was milled in a mortar for one minute and then heattreated at 200° C. for 4 hours whereupon it was observed. Because ofmetal peeling, the outer appearance became brown and the resistivityrose to 75 mΩ-cm.

TABLE 1 (Example 4) point of analysis Si (wt %) P (wt %) Ni (wt %) Au(wt %) A-1 0.00 0.00 14.70 85.30 A-2 0.00 0.00 40.44 59.56 A-3 0.00 1.3598.65 0.00 A-4 0.00 6.93 93.07 0.00 A-5 0.00 11.81 88.19 0.00 B-1 0.000.00 3.60 96.40 B-2 0.00 0.00 5.75 94.25 B-3 0.00 0.00 17.69 82.31 B-40.00 1.81 90.53 7.66 B-5 0.00 3.27 96.73 0.00 B-6 0.00 7.61 92.39 0.00B-7 0.00 9.27 90.73 0.00 C-1 0.00 0.00 13.79 86.21 C-2 0.00 0.00 17.4582.55 C-3 0.00 0.00 46.11 53.89 C-4 0.00 0.00 72.86 27.14 C-5 0.00 0.0078.28 21.72 C-6 0.00 1.34 92.57 6.09 C-7 0.00 1.93 98.07 0.00 C-8 0.002.44 97.56 0.00 C-9 0.00 2.73 97.27 0.00 C-10 14.99 3.59 81.43 0.00 C-1144.60 4.95 50.45 0.00 C-12 100.00 0.00 0.00 0.00

TABLE 2 (Example 5) point of analysis Si (wt %) P (wt %) Ni (wt %) Au(wt %) A-1 0.00 0.00 0.00 100.00 A-2 0.00 0.00 2.05 97.95 A-3 0.00 1.1610.89 87.95 A-4 0.00 6.05 42.98 50.97 A-5 0.00 12.47 75.90 11.63 A-60.00 12.08 78.23 9.69 B-1 0.00 0.00 1.77 98.23 B-2 0.00 0.00 7.70 92.30B-3 0.00 4.45 31.88 63.66 B-4 0.00 9.86 71.63 18.51 B-5 0.00 1O.47 75.9413.59 B-6 0.00 13.35 76.93 9.72 B-7 14.68 14.93 70.40 0.00 B-8 100.000.00 0.00 0.00 C-1 0.00 0.00 2.40 97.60 C-2 0.00 0.00 3.00 97.00 C-30.00 1.49 4.05 94.46 C-4 0.00 2.17 12.24 85.59 C-5 0.00 4.44 31.12 64.45C-6 0.00 5.73 40.70 53.57 C-7 0.00 11.78 72.38 15.84 C-8 0.00 16.6680.26 3.09 C-9 2.48 15.15 81.14 1.23 C-10 9.05 15.07 75.89 0.00 C-1126.44 13.61 59.95 0.00 C-12 100.00 0.00 0.00 0.00

TABLE 3 Example 4 Example 5 Point of P Ni P Ni analysis (wt %) (wt %)P/(p + Ni) (wt %) (wt %) P/(p + Ni) A-1 0 14.70 0.0% 0 0 — A-2 0 40.440.0% 0 2.05 0.0% A-3 1.35 98.65 1.4% 1.16 10.89 9.6% A-4 6.93 93.07 6.9%6.05 42.98 12.3% A-5 11.81 88.19 11.8% 12.47 75.9 14.1% A-6 12.08 78.2313.4% B-1 0 3.6 0.0% 0 1.77 0.0% B-2 0 5.57 0.0% 0 7.7 0.0% B-3 0 17.690.0% 4.45 31.88 12.2% B-4 1.81 90.53 2.0% 9.86 71.63 12.1% B-5 302796.73 3.3% 10.47 75.94 12.1% B-6 7.61 92.39 7.6% 13.35 96.93 14.8% B-79.27 90.73 9.3% 14.93 70.04 17.5% B-8 0 0 — C-1 0 13.79 0.0% 0 2.4 0.0%C-2 0 17.45 0.0% 0 3 0.0% C-3 0 46.11 0.0% 1.49 4.05 26.9% C-4 0 72.860.0% 2.17 12.24 15.1% C-5 0 78.28 0.0% 4.44 31.12 12.5% C-6 1.34 92.571.4% 5.73 40.7 12.3% C-7 1.93 98.07 1.9% 11.78 72.38 14.0% C-8 2.4497.56 2.4% 16.66 80.26 17.2% C-9 2.73 97.27 2.7% 15.55 81.14 15.7% C-103.59 81.43 4.2% 15.07 75.89 16.6% C-11 4.95 50.45 8.9% 13.61 59.95 18.5%C-12 0 0 — 0 0 —

EXAMPLE 6

Preparation of Palladium Colloid-deposited Silica

The PPHS-treated silica of Example 4 would float on the surface of watersince it was hydrophobicized. 100 g of the PPHS-treated spherical silicawas admitted into 50 g of a 0.5% aqueous solution of sodiumdodecylbenzenesulfonate as a surfactant. By agitation, the silica wasdispersed in water within a short time of about 5 minutes.

For palladium treatment, 70 g of a 1% PdCl₂ aqueous solution (0.7 g ofpalladium chloride, 0.4 g of palladium) was added to 150 g of thesilica-water dispersion, followed by 30 minutes of agitation, filtrationand water washing. By this treatment, palladium colloid adhered to thesilica surface, and so the silica was colored blackish gray. Thepalladium colloid-deposited silica was isolated by filtration, washedwith water, and immediately subjected to plating.

Preparation of Copper-plated Silica

The copper plating solution used was 100 g of a dilution obtained bydiluting an electroless copper plating solution C-200LTA/LTB (KoujundoKagaku Kenkyuujo) with an equal volume of ion-exchanged water. Thepalladium colloid-deposited silica particles were dispersed in thecopper plating solution along with 0.5 g of anti-foaming surfactantKS-538 (Shin-Etsu Chemical Co., Ltd.). With vigorous stirring, thesolution temperature was raised from room temperature to 35° C. Finebubbles evolved and the silica turned reddish brown, indicating thatmetallic copper deposited on the silica surface.

The plated silica particles settled on the bottom and were subjected tofiltration, water washing and drying (at 50° C. for 30 minutes) andthereafter, fired at 300° C. for one hour in a nitrogen-purged electricfurnace. The resulting silica particles were observed under astereomicroscope to find that silica particles were covered with copperover their entire surface.

EXAMPLE 7

The sodium dodecylbenzenesulfonate surfactant was omitted, 100 g of thePPHS-treated spherical silica of Example 4 was admitted into 150 g ofwater, and vigorous agitation was continued over 30 minutes fordispersion. For palladium treatment, 70 g of a 1% PdCl₂ aqueous solution(0.7 g of palladium chloride, 0.4 g of palladium) was added to 250 g ofthe silica-water dispersion, followed by 30 minutes of agitation,filtration and water washing. By this treatment, palladium colloidadhered to the silica surface, and so the silica was colored blackishgray.

The palladium colloid-deposited silica was isolated by filtration,washed with water, and immediately subjected to plating in the presenceof surfactant KS-538 as in Example 6. The resultant copper-plated silicahad a firmly bonded metal film.

EXAMPLE 8

Preparation of Palladium Colloid-deposited Silica

100 g of the PPHS-treated spherical silica of Example 4 was admittedinto 50 g of a 0.5% aqueous solution of sodium dodecylbenzenesulfonateas a surfactant. By agitation, the silica was dispersed in water withina short time of about 5 minutes. For palladium treatment, 70 g of a 1%PdCl₂ aqueous solution (0.7 g of palladium chloride, 0.4 g of palladium)was added to 150 g of the silica-water dispersion, followed by 30minutes of agitation, filtration and water washing. By this treatment,palladium colloid adhered to the silica surface, and so the silica wascolored blackish gray. The palladium colloid-deposited silica wasisolated by filtration, washed with water, and immediately subjected toplating.

Preparation of Nickel-plated Silica

The nickel plating solution used was 100 g of a dilution obtained bydiluting an electroless nickel plating solution Ni-901 (Koujundo KagakuKenkyuujo) with a five-fold volume of ion-exchanged water. The palladiumcolloid-deposited silica particles were dispersed in the nickel platingsolution along with 0.5 g of anti-foaming surfactant KS-538 (Shin-EtsuChemical Co., Ltd.). With vigorous stirring, the solution temperaturewas raised from room temperature to 65° C. Fine bubbles evolved and thesilica turned black, indicating that metallic nickel deposited on thesilica surface.

Preparation of Gold-plated Silica (1)

The gold plating solution used was 100 g of an electroless gold platingsolution K-24N (Koujundo Kagaku Kenkyuujo) without dilution. The silicaparticles having metallic nickel deposited on their entire surface weredispersed in the gold plating solution. With vigorous stirring, thesolution temperature was raised from room temperature to 95° C.Immediately thereafter, fine bubbles evolved and the silica turnedgolden, indicating that gold deposited on the silica surface.

The plated silica particles settled on the bottom and were subjected tofiltration, water washing and drying (at 50° C. for 30 minutes) andthereafter, fired at 500° C. for one hour in a nitrogen-purged electricfurnace. The resulting silica particles were observed under astereomicroscope to find that silica particles were covered with goldover their entire surface. On IPC analysis of this silica, palladium,nickel and gold were detected.

EXAMPLE 9

Preparation of Gold colloid-deposited Silica

100 g of the PPHS-treated silica of Example 6 was admitted into 50 g ofa 0.5% aqueous solution of sodium dodecylbenzenesulfonate as asurfactant. By agitation, the silica was dispersed in water within ashort time of about 5 minutes. For gold treatment, 70 g of a 3% NaAuCl₄aqueous solution (2.1 g of sodium chloroaurate) was added to 150 g ofthe silica-water dispersion, followed by 30 minutes of agitation,filtration and water washing. By this treatment, gold colloid adhered tothe silica surface, and so the silica was colored light purple. The goldcolloid-deposited silica was isolated by filtration, washed with water,and immediately subjected to plating.

Preparation of Gold-plated Silica (2)

The gold colloid-deposited silica was dispersed in the above electrolessgold plating solution along with 0.5 g of anti-foaming surfactant KS-538(Shin-Etsu Chemical Co., Ltd.). With vigorous stirring, the solutiontemperature was raised from room temperature to 85° C. Immediatelythereafter, fine bubbles evolved and the silica turned golden,indicating that gold deposited on the silica surface.

The plated silica particles settled on the bottom and were subjected tofiltration, water washing and drying (at 50° C. for 30 minutes) andthereafter, fired at 500° C. for one hour in a nitrogen-purged electricfurnace. The resulting silica particles were observed under astereomicroscope to find that silica particles were covered with goldover their entire surface. On IPC analysis of this silica, only gold wasdetected, but other elements such as palladium, nickel and copper werenot detected.

EXAMPLE 10

Preparation of Palladium Colloid-deposited Silica

100 g of the PPHS-treated spherical silica of Example 6 was admittedinto 50 g of a 0.5% aqueous solution of surfactant Surfynol 504 (NisshinChemical Industry K.K.). By agitation, the silica was dispersed in waterwithin a short time of about 5 minutes. For palladium treatment, 70 g ofa 1% PdCl₂ aqueous solution (0.7 g of palladium chloride, 0.4 g ofpalladium) was added to 150 g of the silica-water dispersion, followedby 30 minutes of agitation, filtration and water washing. By thistreatment, palladium colloid adhered to the silica surface, and so thesilica was colored blackish gray. The palladium colloid-deposited silicawas isolated by filtration, washed with water, and immediately subjectedto plating.

Nickel Plating of Palladium Colloid-deposited Silica

The reducing solution used for nickel plating was 100 g of a mixture of2.0 M sodium hypophosphite, 1.0 M sodium acetate and 0.5 M glycine,diluted with ion-exchanged water. The palladium colloid-deposited silicawas dispersed in the nickel plating, reducing solution along with 0.5 gof anti-foaming agent KS-538 (Shin-Etsu Chemical Co., Ltd.). Withvigorous stirring, the solution temperature was raised from roomtemperature to 65° C. A dilution of 2.0 M sodium hydroxide inion-exchanged water was carried by air and added dropwise, and at thesame time, a dilution of 1.0 M nickel sulfate in ion-exchanged water wascarried by nitrogen gas and added dropwise to the reducing solution.Then fine bubbles evolved and the silica turned black, indicating thatmetallic nickel deposited on the silica surface. The silica particleshad metallic nickel deposited on their entire surface, with noagglomerates or bulbous deposits found.

Gold Plating of Nickel-plated Silica

The gold plating solution used was 100 g of a gold plating solutionK-24N (Koujundo Kagaku Kenkyuujo) without dilution. The silica particleshaving metallic nickel deposited on their entire surface were dispersedin the gold plating solution. With vigorous stirring, the solutiontemperature was raised from room temperature to 95° C. Immediatelythereafter, fine bubbles evolved and the silica turned golden,indicating that gold deposited on the silica surface.

The plated silica particles settled on the bottom and were subjected tofiltration, water washing and drying (at 50° C. for 30 minutes) andthereafter, fired at 300° C. for one hour in a hydrogen-purged electricfurnace. The resulting silica particles were observed under astereomicroscope to find that silica particles were covered with goldover their entire surface. On IPC analysis of this silica, palladium,nickel and gold were detected.

Identification of Conductive Silica having Silica-siliconPolymer-nickel-gold Structure

The gold-plated silica was mixed with an epoxy resin (Araldite A/B),which was cured and sliced. A slice was observed under an electronmicroscope, finding a two-layer structure consisting of a silica portionand a multi-phase plated portion.

The gold-plated silica was analyzed by Auger electron spectroscopy, bywhich constituent elements were analyzed in a depth direction while ionetching the surface. The silica particles were found to have afour-layer structure consisting of a gold layer, a nickel layer, asilicon polymer layer (carbon and silicon-containing layer) and a silicalayer stacked in the depth direction. The silica particles had a yellowouter appearance when observed under a microscope, and a specificgravity of 3.5. The gold layer was 0.03 μm thick, the nickel layer was0.25 μm thick, and the silicon polymer layer was 0.1 μm thick.

Properties of Conductive Silica having Silica-siliconPolymer-nickel-gold Structure

The electric resistivity of the gold-plated silica was determined byintroducing it into a cylindrical cell having four terminals, conductinga current of 1 mA to 10 mA across the terminals of 0.2 cm² area at theopposed ends from a current source SMU-257 (Keithley Co.), and measuringa voltage drop across the 0.2 cm spaced apart terminals at the center ofthe cylinder by means of a nanovolt meter model 2000 (Keithley Co.). Thegold-plated silica had a resistivity of 2.2 mΩ-cm. The gold-platedsilica was milled in a mortar for one minute and then heat treated at200° C. for 4 hours whereupon it was examined to find no change of itsouter appearance and resistivity.

For comparison purposes, in the step of nickel plating on the palladiumcolloid-deposited silica, neither nitrogen gas nor air was used. At theend of plating reaction, agglomerates and bulbous deposits were found.This gold-plated silica had an initial resistivity of 3.2 mΩ-cm. Thesilica was milled in a mortar for one minute and then heat treated at200° C. for 4 hours whereupon it was examined. Metal peeling occurred,the outer appearance turned brown, and the resistivity increased to 75mΩ-cm.

EXAMPLE 11 and Comparative Example 4

Spherical silica (Admafine SO-25H) was treated by either of thefollowing procedures 1 and 2 using the polysilane obtained in SyntheticExample 2 and methylhydrogenpolysiloxane of the following formula.

Procedure 1

Silica was admitted into a 5% THF solution of the polysilane. After onehour, the silica was collected by filtration and dried.

Procedure 2

Silica was admitted into a 5% toluene solution of the polysiloxane.After one hour, the silica was collected by filtration and dried.

By this treatment, the silica was hydrophobicized so that it would floaton the surface of water when poured therein.

Then, 200 parts by weight of a 1% PdCl₂ aqueous solution (2 parts byweight of palladium chloride, 1.2 parts by weight of palladium) wasadded to 100 parts by weight of the silica. Water was evaporated in airto dryness. By this treatment, palladium was borne on the silica, whichwas colored reddish brown or blackish gray.

The palladium-bearing silica was poured into an electroless copperplating solution which was obtained by diluting an electroless copperplating solution C-200LTA/LTB (Koujundo Kagaku Kenkyuujo) with an equalvolume of ion-exchanged water. Immediately after pouring, the silicaremained afloat on the surface. After about 5 minutes, bubbling startedand the silica submerged and settled on the bottom. At this point, boththe solution and the silica became pitch-black. The silica was collectedby filtration and dried. Both the silica treated with the polysilane byProcedure 1 and the silica treated with the polysiloxane by Procedure 2were silica particles covered with a reddish brown copper film on theirsurface.

For comparison purposes, untreated silica was poured into the sameelectroless copper plating solution. The silica settled on the bottomand changed no longer. The silica was collected by filtration and dried,yielding a white powder in which there was admixed a blue-green colorwhich was presumed to be copper sulfate.

The above treated silica was treated in a mixed acid of hydrofluoricacid (HF/HClO₄) at 300° C. to remove the silicon value. Thereafter, themetal components were re-dissolved with nitric acid. Pd and Cu werequantitatively determined by ICP analysis (Shimadzu ICPS-1000). Theresults are shown below.

Polysilane-treated silica:

Pd 1,410 ppm, Cu 9.4 wt %

Polysiloxane-treated silica:

Pd 600 ppm, Cu 4.0 wt %

Untreated silica:

Pd undetected, Cu 0.02 wt %

BENEFITS OF THE INVENTION

There has been described a metallized powder possessing a stronger bondbetween the particle base and the metal even at elevated temperature andexhibiting high conductivity, heat resistance, and conductivitystability. The conductive powder is blended in a rubber composition toprovide a conductive rubber composition, from which conductive rubberparts possessing high conductivity despite a low specific gravity andhaving conductivity stability even at elevated temperature are obtained.The conductive rubber parts may be used as reliable connectors andgaskets.

Japanese Patent Application Nos. 11-132501 and 11-193354 areincorporated herein by reference.

Although some preferred embodiments have been described, manymodifications and variations may be made thereto in light of the aboveteachings. It is therefore to be understood that the invention may bepracticed otherwise than as specifically described without departingfrom the scope of the appended claims.

What is claimed is:
 1. A conductive powder comprising particles eachhaving a surface, an organic silicon polymer layer formed on theparticle surface, and a metal layer enclosing the organic siliconpolymer layer.
 2. A conductive powder comprising particles each having asurface, a partially or entirely ceramic layer formed from an organicsilicon polymer on the particle surface, and a metal layer enclosing theceramic layer.
 3. The conductive powder of claim 1 or 2 wherein saidmetal layer includes a first metal layer and a second metal layerenclosing the first metal layer.
 4. The conductive powder of claim 3wherein said first metal layer is selected from the group consisting ofnickel, copper, silver, cobalt, tungsten, iron and zinc, and said secondmetal layer is selected from the group consisting of gold, platinum, andpalladium.
 5. The conductive powder of claim 4 wherein said first metallayer is nickel, and said second metal layer is gold.
 6. The conductivepowder of claim 3 wherein said particles are of silica.
 7. Theconductive powder of claim 3, wherein the first metal layer has athickness of 0.1 to 2.0 μm and the second metal layer has a thickness of0.01 to 0.1 μm.
 8. The conductive powder of claim 1 or 2 wherein thesilicon polymer layer is made of an organic silicon polymer havingreducing effect.
 9. The conductive powder of claim 8 wherein thereducing silicon polymer is selected from the group consisting of apolysilane, polycarbosilane, polysiloxane, and polysilazane having Si—Sibonds and/or Si—H bonds.
 10. The conductive powder of claim 9 whereinsaid polysilane is represented by the following general formula (1): (R¹_(m)R² _(n)X_(p)Si)_(q)  (1) wherein R¹ and R² each are hydrogen or asubstituted or unsubstituted monovalent hydrocarbon group; X is a groupas defined for R¹, alkoxy group, halogen atom, oxygen atom or nitrogenatom; m, n and p are numbers satisfying 0.1≦m≦2, 0.1≦n≦1, 0≦p≦0.5, and1≦m+n+p≦2.5, and q is an integer of 4≦q≦100,000.
 11. The conductivepowder of claim 9 wherein said polysiloxane is represented by thefollowing general formula (2): (R³ _(a)R⁴ _(b)H_(c)SiO_(d))_(e)  (2)wherein R³ and R⁴ each are hydrogen, a substituted or unsubstitutedmonovalent hydrocarbon group, alkoxy group or halogen atom; a, b, c andd are numbers satisfying 0.1≦a≦2, 0≦b≦1, 0.01≦c≦1, 0.5≦d≦1.95, and2≦a+b+c+d≦3.5, and e is an integer of 2≦e≦100,000.
 12. A conductivepowder comprising particles each having a surface, and an organicsilicon polymer layer, a nickel-phosphorus alloy layer, and a gold layerformed successively on the particle surface, wherein saidnickel-phosphorus alloy layer has a phosphorus content which differsbetween inner and outer surface regions.
 13. The conductive powder ofclaim 12, wherein the inner surface region of the nickel-phosphorusalloy layer has a high phosphorus content and the outer surface regionof the nickel-phosphorus alloy layer has a low phosphorus content. 14.The conductive powder of claim 13, wherein the inner surface region ofthe nickel-phosphorus alloy layer has a phosphorus content of 5 to 20%by weight and the outer surface region of the nickel-phosphorus alloy.layer has a phosphorus content of 0.5 to 8% by weight.
 15. A conductivepowder comprising particles each having a surface, and a partially orentirely ceramic layer of an organic silicon polymer, anickel-phosphorus alloy layer, and a gold layer formed successively onthe particle surface, wherein said nickel-phosphorus alloy layer has aphosphorus content which differs between inner and outer surfaceregions.
 16. The conductive powder of claim 15, wherein the innersurface region of the nickel-phosphorus alloy layer has a highphosphorus content and the outer surface region of the nickel-phosphorusalloy layer has a low phosphorus content.
 17. The conductive powder ofclaim 16, wherein the inner surface region of the nickel-phosphorusalloy layer has a phosphorus content of 5 to 20% by weight and the outersurface region of the nickel-phosphorus alloy layer has a phosphoruscontent of 0.5 to 8% by weight.